Large bearings, and particularly large rolling bearings, are typically submitted to heavy loads. Such heavy loads may be imposed to these bearings either in stress conditions or at rest. Although each bearing may be appropriately selected and dimensioned for a particular heavy load application, proper lubrication may generally be regarded as an important factor for its overall performance during the bearing's estimated service life.
Although rolling bearings often work well in non-ideal conditions, sometimes minor problems may cause bearings to fail quickly and unpredictably. For example, under a stationary (non-rotating) load, small vibrations can gradually press out the lubricant (or grease) between the races and rollers or balls of the bearing. This situation is known as false brinelling. To avoid false brinelling, accurate and local lubrication is required, particularly in the area where the load transmission between rolling elements and raceways takes place. The quality of the lubricant is also of utmost importance in order to prevent failure modes. As a consequence, old lubricant needs to be removed at a regular basis in order to maintain sufficient lubrication properties.
Another factor that affects performance of a bearing is the volume of lubricant in the bearing. Typically, bearings are delivered filled with their corresponding lubricant at an ideal percentage, which in some applications may be about 60% of their free volume. Regular lubrication cycles intend to maintain the volume of lubricant inside the bearing close to that ideal percentage. However, after a significant operating time, the volume of lubricant inside the bearing may divert from the ideal percentage.
If the volume of lubricant falls short of the ideal percentage, then the hydro dynamic lubricant layer may become thin or disappear in certain points or areas. Consequently, the friction at the load transmission areas may increase and so does the risk of wear initiation. Furthermore, the increased friction increases in turn the temperature of the remaining lubricant. Thus, the lubricant may suffer accelerated aging damage which may affect its performance. In extreme cases, damaged additives, soaps and oil may even accelerate the initiation of fatigue signs inside the bearing.
On the other hand, bearings working with a level of lubricant above the ideal level may be exposed to local excess pressure inside the component due to the combined effects of accumulation of lubricant close to the bearing inlets and rolling motion of the balls or rollers in the same area. An overpressure inside a bearing may lead to failure of the sealing means. For example, in case of 4 points contact slew bearings, rubber seals may pop out of their housing and leave the component exposed to lubricant leakage and external agent contamination. This last consequence may be critical for the service life of bearings since it may generally lead to indentation effects or corrosion of the raceways in case water or solid contaminants enter inside the bearing. The final effect may be the requirement for substitution of the component well before its expected service life.
The above problems are of particular importance in large bearings in the wind energy sector. Wind turbine generators employ large bearing assemblies, such as the main, pitch and yaw bearing assemblies that are subject to particularly heavy loads. Furthermore, the orientation of the bearings, with respect to, for example, the wind turbine rotor blade, may lead to high concentrations of lubricant in certain areas during periods of low or no wind. For example, in a pitch bearing assembly of a wind turbine generator, certain zones of the bearings may suffer from lubricant concentration and the bearings may potentially be affected by an internal overpressure at these zones. One such zone may be the bearing zones in the rotor plane at the trailing edge of the blades of the wind turbine. A combination of gravity and centrifugal forces may cause concentration of lubricant and overpressure in the affected zones.
To mitigate these problems, manufacturers of lubricant systems have developed specific products with the aim of bringing fresh lubricant to the bearing at reduced intervals. Control systems of lubrication systems range from local fixed intervals lubrication control solutions to external lubrication control solutions. In the latter case, controllers may also manage when and how much lubricant is applied to the bearing.
With respect to the lubricant recovery, some bearings' designers suggest closed outlets for keeping the lubricant inside the bearing and a passive extraction of the lubricant only during maintenance operations. In some alternative solutions, individual deposits may be fixed to the bearing outlets for a continuous passive recovery of the lubricant. These deposits may be emptied or replaced during maintenance operations.
Finally, active systems have been developed for the used lubricant recovery. In some cases, lubricant may be extracted from the bearing outlets, while in others a small intermediate deposit may be connected to the bearing outlet with a suction element which extracts the lubricant from these deposits.
In wind turbine generators, the most advanced existing solutions offer control of the lubricant injection only, depending on the working conditions of the bearing. In these cases, the lubricant extraction from the bearing is carried out by active systems which depend on the lubricant injection.
However, although the existing lubrication control systems may be able to monitor precisely the amount of injected lubricant, the amount of remaining lubricant inside the bearing remains out of their scope. Furthermore, the distribution of lubricant inside the bearing remains unknown.